The present invention relates to electrochemical capacitors wherein reliability and durability are improved by reducing the pressure generated by change in volume accompanying charging and discharging, when high-performance carbon materials with large electrostatic capacity are used as polarizing electrodes. The present invention also relates to electrostatic capacitors having structures relaxing the stress generated by expansion of the polarizing electrodes on charging.
Besides being used as back-up power sources and batteries for transportation vehicles from automobiles down, as they have large capacity of the Farad class and are excellent in charging and discharging cycle characteristics, electric double layer capacitors, a type of capacitors, are investigated for use in off-peak power storage from the viewpoint of effective utilization of energy.
A single electrode cell 10, one of the basic structures of such electric double layer capacitors, has the structure as. shown in FIG. 15 in which a positive side polarizing electrode 24 and a negative side polarizing electrode 26 are respectively formed on collectors 20 and 22 generally comprised of metallic materials, said polarizing electrodes 24 and 26 being separated by a separator 28, and are impregnated with electrolytic solution comprising the solvent and electrolytes.
FIG. 16 shows the structure of a single capacitor cell 12, wherein plurality of single electrode cells 10 are electrically connected parallel with each other to electrode outlet parts 30 and 32 formed on collectors 20 and 22. Such a single capacitor cell 12 is suitably used as an electric double layer capacitor with relatively large capacity used for automobiles, etc. Both single electrode cells 10 or single capacitor cells 12 are planar and characterized by ease of tight packing and expansion.
In contrast to such planar electric double layer capacitors, there are also coil type electric double layer capacitors 70 having structures suitable for capacity enlargement similar to a single capacitor cell 12 as shown in FIG. 17. A coil type electric double layer capacitor 70 is prepared using a coil 76 prepared by cylindrically coiling a positive electrode sheet 72, in which a positive electrode side polarizing electrode 24 is formed on a collector 20, and a negative electrode sheet 74, in which a negative pole side polarizing electrode 26 is formed on a collector 22, with a separator 28 between them, and for example by putting said coil 76 in a case 78 and filling the case with electrolytic solution, and sealing the opening end surface of the case 78 with a sealing plate 82 having an electrode terminal 80 formed on it, while securing continuity between electrode sheets 72 and 74 and each electrode terminal 80.
As polarizing electrode materials for such electric double layer capacitors, those having activated carbon with relative surface area of 1000 m2 or more as a main ingredient have traditionally been used. The construction of a pair of electrodes of an electric double layer capacitor using such activated carbons will be explained referring to FIG. 18.
As shown in FIG. 18, this pair of electrodes 1 is constructed to have a positive pole 2 and a negative pole 3 stuck to one another with a separator 6 between them.
To explain more precisely about the positive pole 2 and the negative pole 3, they have layers of polarizing electrodes 5 containing activated carbon as an essential ingredient and optionally binders such as polytetrafluoroethylene and conduction aids such as carbon black, on the surface of a collector 4 comprising aluminum foil, etc.
Further, the layer of polarizing electrode 5 is formed for example by blending a mixture of fixed ratio of activated carbon, a binder and a conduction aid to give a sheet of fixed shape and area, which is then adhered onto a collector 4. It may also be formed by coating the surface of a collector 4 with a paste of a mixture of activated carbon, a binder, a conduction aid, and a solvent and drying it to remove the solvent by evaporation. Known coating methods such as spray coating and brush coating may be adopted.
Here the separator 6 is impregnated with an organic electrolytic solution (not shown) such as ethylene carbonate, propylene carbonate, xcex3-butyrolactone, and sulfolane, containing a fixed concentration, for example, of 1 mol/l of an electrolyte such as Et4NBF4 (Et4N; tetraethylammonium), Et4NPF6, Bu4NBF4 (Bu4N: tetrabutylammonium) or Bu4NPF6.
When such an electric double layer capacitor having a pair of electrodes 1 and a separator 6 is charged, the ions of the electrolyte are adsorbed inside micropores of the activated carbon which is the essential ingredient of the polarizing electrode, and this adsorption causes capacitance to occur in the electric double layer capacitor.
As the separator 6, materials may be used that electrically insulate the positive pole 2 and the negative pole 3 to prevent them to short, and may allow ions of the electrolytes to penetrate so that ions of the electrolytes can migrate toward the positive pole and the negative pole 3 during charging and discharging. Cellulose blend paper is particularly suitable. Cellulose blend paper also has advantages that it is inexpensive, enabling cost reduction, and may be formed thinner, enabling reduction of internal resistance of the electric double layer capacitor.
Further, cellulose blend paper will not melt even when the positive and negative electrodes short, as it has a high melting point. Therefore, it is seldom used for electric cell separators. This is because migration of electrolyte ions is not prevented, as ion penetrating pores of the separator are not blocked even when the internal temperature of an electric cell rises due to excessive reaction of the cell when electrodes short, resulting in further increase of the internal temperature.
Internal temperature of an electric double layer capacitor, however, scarcely rises even when the separator does not melt in a short circuit. In other words, there is no problem in using high melting point separators, since internal temperature would not rise significantly in a short circuit. These and the advantages described above are the reason why cellulose blend paper is widely used.
Capacitance and upper limited voltage of conventional electric double layer capacitors described above depend on the activation method of activated carbon. For example, capacitance is 15 F/cc and upper limited voltage is 3 V for an electrochemical capacitor using steam activated carbons, and capacitance is 20 F/cc and upper limited voltage is 2.5 V for an electrochemical capacitor using alkali-activated carbons.
Capacitance and upper limited voltage of various capacitors including electrical double layer capacitors are expected recently to improve further, those with these values above 30 F/cc and 3.5 V being desired.
Thus, alkali-activated new polarizing electrode materials are reported in Japanese patent provisional publications No. 275042/1997 and 320906/1997 in order to increase the capacity of polarizing electrode materials.
In the case of electrochemical capacitors using alkali-activated polarizing electrode materials as disclosed in Japanese patent provisional publications No. 275042/1997 and 320906/1997, however, there is a problem of volume expansion of the polarizing electrode materials due to charging. There was no such problems of volume expansion in electric double layer capacitors using conventional activated carbons.
This volume expansion tends to increase as the amount of electricity contributing to charging and discharging increases, and is especially a big problem in uses wherein it is preferable that capacitance is big, charging and discharging capacity is big, and further capacitance density (density of capacitance for unit volume) or capacitance weight-density (density of capacitance for unit weight) is big, especially as in automobiles.
For example, in electrochemical capacitors using single capacitor cells 12 described above, single capacitor cells 12 are tightly packed in a capacitor case (to be called xe2x80x9ccasexe2x80x9d hereafter) with least opening in order to make the capacitance density as large as possible. Here electrode outlets 30, 32 are formed in the direction perpendicular to the direction of stacking of single electrode cells 10 -constructing single capacitor cells 12, separators 28 are designed to have areas larger than those of polarizing electrodes 24, 26, and the polarizing electrodes 24, 26 expand little in the direction perpendicular to the direction of stacking. Therefore polarizing electrodes 24, 26 do not hardly touch the internal walls of the case directly, so that they are not susceptible to volume expansion described above.
In the direction of stacking of single electrode cells 10 in single capacitor cells 12, however, the surface pressure from volume expansion of polarizing electrodes 24, 26 of each single electrode cell 10 sums up to a significant amount of pressure, possibly causing deformation or destruction of the case and damage or deterioration of collectors 20, 22 and separators 28, eventually even inducing deterioration of the cycle characteristics. Though it is possible here to deal with the surface pressure by employing stronger or thicker materials for the case, this may cause reduction of capacitance density or weight density of capacitance.
On the other hand, when holding pressure between internal walls of the case and single capacitor cells is reduced to lessen the surface pressure due to such volume expansion, the single capacitor cells 12 may not be held stably in the case, causing problems of increased internal resistance and greater deviation of capacitance, that would reduce stability of properties. Such problems would be especially greater under circumstances where there is constant vibration as in the case of being installed in automobiles.
As coil-type electric double layer capacitors 70 can be considered to have structures wherein polarizing electrodes are stacked in the direction of the core, the problem of volume expansion of polarizing electrodes in the electrochemical capacitors having a coil-type structure may be considered in the same manner as in the case of single capacitor cells 12. Thus, in coil-type electrochemical capacitors, surface pressure is applied onto the cylindrical wall of the case, that would cause deformation or destruction of the case with greater possibility.
The present invention was made in view of the problems of conventional technology described above, with the purpose of providing electrochemical capacitors in which reduction of surface pressure is enabled without reducing capacitance by paying attention to electrolytes of organic electrolytic solutions used in electrochemical capacitors.
Another purpose of the present invention is to provide electrolytic capacitors showing capacitance and upper limited voltage that are superior to those of electric double layer capacitors using activated carbon.
Thus, according to the present invention, there is provided an electrochemical capacitor comprising:
polarizing electrodes, whose main ingredient is a carbon material having partially oxidized graphite-like microcrystalline carbon, and
an organic electrolytic solution,
said polarizing electrodes being immersed in the organic electrolytic solution, and said polarizing electrodes expanding on charging and contracting on discharging,
wherein ions with radius of ions of 0.33 nm or less are used as electrolyte cations of said organic electrolytic solution.
In such electrochemical capacitors of the present invention, it is preferable to use trimethylammonium ion or methylethylpyrrolidinium ion as the cation.
Also, according to the present invention, there is provided an electrochemical capacitor comprising:
electrode members comprised of polarizing electrodes, whose main ingredient is a carbon material having partially oxidized graphite-like microcrystalline carbon, separators, and collectors,
an organic electrolytic solution, and
a cell container containing the electrode members and the organic electrolytic solution,
said electrode members being immersed in the organic electrolytic solution, and said polarizing electrodes expanding on charging and contracting on discharging,
wherein said electrode members and said cell container are provided with stress relaxing structure.
As stress relaxing structures suitably employed in the electrochemical capacitors of the present invention, there are bellows and flat spring types. Further, it is also preferable to use an elastomer for either the collector or the separator.
According to the present invention, there is further provided an electrochemical capacitor comprising:
electrode members comprised of polarizing electrodes, whose main ingredient is a carbon material having partially oxidized graphite-like microcrystalline carbon, separators, and collectors, and
an organic electrolytic solution, said electrode members being immersed in the organic electrolytic solution, and said polarizing electrodes expanding on charging and contracting on discharging,
wherein at least either said collectors or said separators is an elastomer.
It is preferable to use elastically deformable porous conductive rubber sheets or porous aluminum sheets as collectors for electrochemical capacitors according to the present invention.
In the electrochemical capacitor described above, it is preferable to use an elastically deformable insulating porous body as the separator, and one formed of porous rubber, porous cloth or nonwoven fabric of glass fiber, and cellulose nonwoven fabric as the insulating porous body. Also, the electrode structure is preferably in an oppressed condition under the pressure of 2 kgf/cm2 (196 kPa) or more.
Plastic deformation of the cell containers will not occur in these electrochemical capacitors of the present invention, as generated stress is relaxed by the cell containers or the electrode structures.
According to the present invention, there is still further provided an electrochemical capacitor comprising:
polarizing electrodes, whose main ingredient is a carbon material having partially oxidized graphite-like microcrystalline carbon,
separators, and
an organic electrolytic solution,
said polarizing electrodes and said separators being immersed in the organic electrolytic solution, and said polarizing electrodes expanding on charging and contracting on discharging,
wherein said separators are sponge-like porous insulators with continuous pores.
In such electrochemical capacitors, cloth or nonwoven fabric having at least inorganic fibers is preferably used as the porous insulators. It is also preferable to use polymeric materials as the porous insulators.
The electrochemical capacitors will not run short of organic electrolytic solution by using, as the separators, sponge-like materials that can be impregnated with large amounts of organic electrolytic solution. Further, as these separators are easily deformable, polarizing electrodes will not go through the separators even when expanded. Thus, the positive and negative poles may not short.